fix: bound mapper cache and reuse host regexes

[umputun]

Reported by @paskal. mappersCache had no size limit and no eviction, and was written for every distinct host reaching the pattern-matching path. A server key that looks concrete is not treated as one: findMatchingMappers compiles each non-default key as an unanchored regex, so a configuration whose only server is example.com still caches every host containing that substring. The cache is written before any path matching, auth or throttling runs, so a request that ends in 502 still costs a permanent entry.

The cache is now a fixed 1024-entry LRU, and each server key's regex compiles once when the mapper table is installed rather than on every cache miss. Without that second half a capped cache would just move the attack from memory to CPU.

Measured with distinct Host values against a wildcard route, RSS delta immediately after each run:

              50k hosts    150k hosts
master          17.2 MB       41.1 MB
branch           8.7 MB       10.0 MB

Master's retention scales with the request count; here it does not, and the residual is transient allocation rather than retained entries.

Exact, wildcard, catch-all and unanchored-regex matching are all unchanged, and no metrics behaviour changes. Server keys are deliberately not anchored: that would change route matching for every existing configuration.

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